I don't think Rossi, in his mind, has anything to prove on the Ducati at the end of this season. He gave it 2 years, and apparently they couldn't improve the bike enough for him to be competitive (like I'm sure he hoped in the beginning).
Of course, next year, on the Yamaha, we will find out if he still IS competetive.
"Nature never hurries, yet everything is accomplished."
- Lao Tsu